In the rat, liver and intestine were found to be the only sources of the constituent apoproteins of plasma lipoproteins. Intestine produced 19% of the total apolipoprotein pool -- 50-60% of apolipoprotein A-I (apoA-I) and apoA-IV, 16% of apoB, less than 10% of apoC and less 1% of apoE. The quantitative order of synthesis in the intact animal was apoE greater than apoB greater than apoA-I greater than apoC greater than apoA-IV. ApoA-I and apoA-IV are major constituents of plasma high density lipoproteins, and so intestine contributes importantly to this lipoprotein class. In other studies with in situ autoperfused jejunal segments from fed rats, plasma glutamine was identified as the major respiratory fuel after the lumen was emptied. Glutamine nitrogen was released into portal blood as ammonia and alanine, substrates for urea synthesis in liver, and as citrulline, a substrate for arginine synthesis in kidney. During absorption of a typical meal, intestinal metabolism of luminal glutamine, glutamate and aspartate accounted for 40% of the respiratory CO2 produced. These studies reveal the importance of glutamate and aspartate in intestinal energy metabolism and the quantitatively significant role of small intestine in processing circulating glutamine, a major nitrogenous end-product from muscle and other tissues.